This invention relates to a servo system in a disc-drive for accessing selected data tracks on disc surface, and more particularly, this invention relates to a servo system wherein a first continuous position signal is generated, and a second position signal is generated from position information stored in sectors of disc surfaces also having data information thereon.
In data storage apparatus of the type using a stack of rotating discs as the recording medium, data is usually recorded in a plurality of concentric data tracks on the surfaces of the discs. The tracks are closely spaced, for example, at 300 tracks per inch, and the data recorded at densities in excess of 5,000 bits per inch. In order to access the recorded data, it is necessary to move a record and playback head (or transducer) to the selected track in which the desired information is stored, and maintain the head precisely over the center of the track with a minimum displacement error in the presence of any possible displacement error for the whole time the information is being read or being written. A typical prior art technique is to dedicate one entire surface of a single disc in the stack of discs to pre-recorded track positioning information. A read-only servo head is associated with said pre-recorded servo disc surface. The read-only head servo head is ganged with other recording and playback heads (data heads) that are associated with the data discs that form the remainder of the stack. As the discs are rotated, a continuous position signal is read from the read-only servo head to indicate the radial position of the data heads relative to the desired data track on the data discs.
In an improvement over the aforementioned prior art system, employing a dedicated servo head and a dedicated servo disc in conjunction with other data heads and data discs, U.S. Pat. No. 4,072,990 additionally employs the technique of embedding additional servo information in sectors of the data discs that contain data information. That patent teaches that while continuous positioning signal from the dedicated servo disc provides high frequency information, other low frequency disturbances, for example, the differential thermal expansion effects between servo head and servo disc and between data heads and data discs produced by change in the ambient temperature, must be detected and corrected. The use of dedicated servo information in sectors of data discs provide such low frequency information. The teaching of that patent is to combine the high frequency information derived from the continuous positioning signal and the low frequency information derived from the intermittent positioning signal generated from servo information embedded in sectors of data discs. For U.S. Pat. No. 4,072,990 the combination of the signal from the continuous servo surface and the information from the sectors of the data discs is achieved by simply providing a low-pass filter for extracting low frequency component from the signal from the sector information and a high-pass filter for extracting high frequency component of the continuous information derived from the dedicated servo disc. The two signals are then added together.